An aim of power line communications technology is to transmit digital data by exploiting the existing infrastructure of the electrical network. In particular, it is possible to remotely read electrical meters, allow for exchanges between electric vehicles and recharging terminals, or even allows for management and control of energy networks (smart grid).
Power line communications (PLC) technology notably incorporates narrowband power line communications (N-PLC) which is generally defined as a communications over an electrical line operating at transmission frequencies up to 500 KHz. The N-PLC communications thus generally uses frequency bands notably defined by the European Committee for Electrotechnical Standardization (CENELEC) or by the Federal Communications Commission (FCC).
Thus, to consider the CENELEC A frequency band (3-95 kHz), the transmission frequencies are situated between 42 and 89 KHz in the PRIME standard, whereas they are situated between 35 and 91 KHz for the PLC-G3 standard.
The signals used in PLC communications are signals modulated according to a multicarrier modulation, for example, a quadrature modulation on orthogonal carriers (Orthogonal Frequency Division Multiplexing modulation, or OFDM modulation), but using only a subset of carriers out of a larger set of available carriers.
Thus, for example, to consider the CENELEC A frequency band, the size of the inverse Fourier transform and of the direct Fourier transform is equal to 512, whereas only 97 sub-carriers (the sub-carriers 86 to 182) are used for the transmission in the PRIME standard.
To consider the CENELEC A frequency band, the size of the inverse Fourier transform and of the direct Fourier transform is equal to 256 while only 36 sub-carriers (the sub-carriers 23 to 58) are used in the PLC-G3 standard.
The signals used in PLC communications and modulated according to OFDM modulation exhibit a crest factor greater than one and generally higher. The crest factor of a signal, commonly called PAPR (Peak-to-Average Power Ratio), is a characteristic measurement of this signal. It is the ratio between the absolute value of the maximum amplitude of the peaks of the signal and the effective signal value. It is equal to one for a constant signal, and greater than one as soon as the signal exhibits peaks.
In PLC communications, the impedance of the communications channel (the electrical line) seen by the transmitter can vary during communications and can drop when a user connects any device such as, for example, a hairdryer or a washing machine.
Typically, a resistive impedance of 2 Ohms seen by the transmitter serves as a reference for determining the maximum output power of the transmitter. Now, depending on the number of devices connected to the electrical line, this impedance seen by the transmitter may be less than 2 Ohms, or even lower.
Also, when the transmitter transmits a signal in a line having an impedance less than 2 Ohms, the power amplifier of the transmitter will enter into saturation in terms of current. The amplifier then enters into a current limiting mode in which it clips the current peaks exceeding the authorized maximum current. The result thereof is then a distortion of the signal and a generation of noise harmonics, or harmonic interferers.
Since the transmission frequencies of the signal are situated between 42 and 89 KHz in the PRIME standard and between 35 and 91 KHz for the PLC-G3 standard, the second harmonics are situated between 70 KHz and 180 KHz.
Consequently, some of these harmonics interfere with the upper part of the useful frequency band of the signal. Furthermore, these harmonics provoke interferences outside of the useful band of the signal which can disrupt other equipment.
Moreover, when the transmitter has to satisfy the requirements of the EN50065-1 standard, which is the case for transmissions according to the PRIME and PLC-G3 standards, the level of the output signal of the transmitter is measured with a peak detector over a pass band of 200 Hz and no part of the spectrum of the transmitted signal must exceed 120 dBμV.